ГЕОХИМИЯ, 2012, № 8, с. 764-783
PETROGRAPHY AND GEOCHEMISTRY OF EOCENE SANDSTONES
FROM EASTERN PONTIDES (NE TURKEY): IMPLICATIONS FOR SOURCE AREA WEATHERING, PROVENANCE AND TECTONIC SETTING
© 2012 г. Cigdem Saydam Eker
Department of Geology, Engineering Faculty, Gumushane University, 29000 Gumushane, Turkey; e-mail: csaydam@gumushane.edu.tr, csaydam@hotmail.com Поступила в редакцию 09.04.2010 г.
Принята к печати 27.04.2011 г.
Subaerial weathering level, source area and tectonic environments were interpreted by using petrographic and geochemical characteristics of Eocene age sandstones found in the eastern Pontides. The thickness of Eocene age clastic rocks in the eastern Pontides ranges from 195 to 400 m. Mineralogical components of sandstones were mainly quartz, feldspar, rock fragments, and opaque and accessory minerals. Depending on their matrix and mineralogical content, Eocene age sandstones are identified as arkosic arenite — lithic arenite and feld-spathic wacke — lithic wacke. CIA (Chemical Index of Alteration) values observed in the Eocene age sandstones (43—55) suggest that the source terrain of the sandstones was not affected by intense chemical weathering. Low CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values of the sandstones studied here suggest only slightly decomposed material and having undergone little transport until final deposition. Zr/Hf, Th/Sc, La/Sc and CIA ratios are low and demonstrate a mafic source; on the other hand, high LREE/HREE ratios and a slightly negative Eu anomaly indicate a subordinate felsic source. Modal mineralogical and SiO2/Al2O3 and K2O/Na2O and Th, Zr, Co, Sc of Eocene age sandstone contents indicate that they are probably magmatic arc originated and deposited in the back arc basin.
Key words: Eastern Pontides, sandstone provenance, source area, tectonosedimentary environment.
1. INTRODUCTION
Eocene aged clastic rocks of eastern Pontides (NE Turkey) exhibit two different source characteristics. Volcaniclastic deposits are dominant in the northern section of Gumushane and siliciclastic deposits are dominant in the southern section (Kelkit, Kose). In Bayburt, deposition starting with basal conglomerate has volcaniclastic characteristic in north of Viricna village and has siliciclastic characteristic in other sections (Fig. 1). In the study area, a total of 3 stratigraph-ic sections were measured, of which 2 were from silici-clastic deposition and 1 from volcaniclastic deposition. Thickness ofvolcaniclastic series measured around Eski Gumushane (Gumushane) is about 195 m, 280 m around Kelkit (Gumushane) [I] and 400 m around Maden (Bayburt) regions (Fig. 2). Objectives of this study are to determine petrographic and geochemical characteristics of Eocene age sandstones in eastern Pontides and to explain their implications on subaerial weathering, source area and tectonic environment.
2. GEOLOGIC SETTING
The northern part of Turkey was described as a Pontides tectonic unit by Ketin [2], which was subse-
quently divided as western and eastern Pontides by S engor and Yilmaz [3] Many geological studies have been conducted in eastern Pontide for various reasons. In addition, Korkmaz et al., Gedik et al., Okay and S ahinturk, and Guven et al. [4—7] studied the general stratigraphic features of the Pontide. The main geological properties of the eastern Pontide are briefly described below.
The basment rocks of the eastern Pontide consist of metamorphic and granitoids intruding the metamor-phic rocks. Liassic volcanics, volcaniclastic, and clastic deposits lie disconformably on this basement rocks. This unit is overlain by pelagic and neritic carbonates of Malm-Lower Cretaceous age. The very thick units of Upper Cretaceous age lie disconformably over the carbonates. The Upper Cretaceous, largely represented by volcanics in the north, has developed into a tur-biditic facies in the south [8], Eocene age rocks in Gu-mushane region are composed of volcanic, volcano-sedimentary and siliciclastic rocks and called as Alibaba Formation [9]. This formation is widespread in the region and lies over the bottom rocks with discordance (Fig. 1). Eocene age rocks in Bayburt region are composed of Danisment Volcaniclastic, basal conglomerate and turbiditic members and called as Erkeli For-
Fig. 1. Simplified geological map of the eastern Pontides [7] and location map of the study area. 1 — Paleozoic metamorphic basement, 2 — Paleozoic granites, 3 — Jurassic-Lower Cretaceous volcano-sedimentary sequences and platform carbonates, 4 — Upper Cretaceous volcanics, 5 — Upper Cretaceous sedimentary rocks, 6 — Paleocene volcano-sedimentary sequences, 7 — Pale-ocene granites, 8 — Eocene volcanic, volcanoclastic rocks, 9 — Eocene sedimentary rocks, 10 — thrust fault, 11 — study area (1 Eski Gumushane section, 2 Kelkit section, 3 Maden section).
mation [10]. Erkeli Formation lies over the bottom units with discordance. Miocene and Pliocene deposits occur in restricted areas and are characterized by clastic material.
3. METHODS
Three stratigraphic sections were measured from the study area. While measuring stratigraphic sections, 125 sandstone samples were collected for sedimentary
petrographic analysis. Among the petrographic analyzed samples, 31 of them (Eski Gumushane (Gumus-hane) section 6, Kelkit (Gumushane) section 10, Maden (Bayburt) section 15) were selected for modal and geochemical analysis (Fig. 2). Modal analysis was carried out by using Gazzi-Dickinson method [11]. In this method, if the size of minerals contained by a large rock fragments seen in thin section is over 0.0625 mm, this mineral is counted as grain or crystal and 1000 grain counts were realized for each sample.
Bulk-rock chemical composition of 31 samples was determined partly by inductively coupled plasma-mass spectrometry (ICP-MC) at the commercial Acme Analytical Laboratories Ltd. in Vancouver, Canada. In the ICP-MS analyses, a 0.2 g aliquot is weighed in to a graphite crucible, placed in an oven and heated to 1050°C for 15 minutes. The molten samples are dissolved in 5% HNO3 (ACS-grade nitric diluted in dem-ineralized water). Calibration standards and reagent blanks are added to the sample sequence. Sample solutions are aspirated into an 1CP emission spectrograph (Jarrel Ash Atom Comb 975) for determining major oxides and certain trace elements (Ba, Nb, Ni, Sr, Sc, Y & Zr), while the samples solutions are aspirated into an ICP mass spectrometer (Perkins-Elmer Elan 6000) to determine trace elements, including rare-earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Br, Tm, Yb, and Lu).
4. LITHOLOGY AND SEDIMENTOLOGY FEATURES
Eocene age volcaniclastic deposit lies over Upper Cretaceous units with turbiditic characteristics in Gu-mushane region with discordance. Stratigraphic section of Eski Gumushane (Gumushane) was measured around Venk Yaylasi.
Based on data obtained from field surveys, deposit starts with grey colored thick layered limestone at the base and is sequenced upward with thin-thick layered, dark grey, brown colored sandstones, thin-thick layered grey, green, yellowish brown colored marl-clay-stone and tuff [1], Eocene age sedimentary rocks in Kelkit (Gumushane) region are composed of silici-clastic turbidities. Deposit comes over Cretaceous age turbiditic characteristic rocks of this region with discordance. Kelkit (Gumushane) measured stratigraphic section was taken around north of Kelkit town.
Field surveys indicate that the unit in the region starts with grey colored, coarse grained, medium layered sandstone and similar characteristics continue up to 9th m. At the 9th m, there is a 2 m thick gravelstone
layer with 1—5 cm diameter circular boundary grains with generally calcite cements and this layer has a different lithology. This rock does not have lateral continuity and is probably a canal fill. There comes a very thin-medium layered yellowish brown colored sandstone over it. It is then sequenced upward with thin-thick layered, light grey, brown colored sandstone, and thin, medium layered, green, greenish grey, yellowish brown marl. Partial grading and parallel lamination are also observed as sedimentary structure in the deposit [1]
In Maden (Bayburt) region, Eocene age siliciclas-tic rocks lies with discordance over Maastrichtian (Late Cretaceous) age limestones. Maden (Bayburt) measured stratigraphic section was taken around northeast of Maden. Based on field surveys, this unit starts with grey, brown colored, medium layered, fine and medium grained sandstone and then grey, grayish green, thin, medium layered marl. The unit continues with sequencing of medium, thick layered sandstone and marl. In this way, layer thickness changes continuously. There are plant residues in the base sections of the deposit, flute marks, rill marks, load casts and grading in thick layered sandstones. The unit ends with dark grey marl-shale sequence (Fig. 2).
5. PETROGRAPHY
Texture. Eocene age sandstones have generally poor, medium sized and grains have shapes changing from angular to round. Matrix percentages were different in Eski Gumushane (Gumushane), Kelkit (Gu-mushane) and Maden (Bayburt) sections and were 4.7-45.5%, 0-0.4%, 1.7-42%, respectively. Cement percentages were 1.6-30.4%, 9.8-26.8%, 3.2-24.9%, respectively for Eski Gumushane (Gumushane), Kelkit (Gumushane) and Maden (Bayburt) sections. Type of cement was generally calcite.
Grain type. Relative amounts of components were determined by performing petrographic modal analysis of sandstone samples taken from the study area (Table 1). Sandstones were mainly composed of quartz (monocrystalline quartz, polycrystalline quartz), feldspar (alkaline feldspar and plagioclase), rock fragments (metamorphic rock fragment, volcanic rock fragment, and sedimentary rock fragment), opaque mineral and accessory mineral (epidote, chlorite, oli-vine, biotite and muscovite).
Eski Gumushane (Gumushane) Section. Quartz (Q): Monocrys
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.